If you can't wait, need them now and have an ebay account:
http://cgi.ebay.com/MacBook-Air-13-3-A12...
This is the entire screw set even thou you just need the bottom, not sold that way at this time.
If not you can check with your local laptop repair places that service macs.
Screws shouldn't come lose, i don't know if this is a problem for macbook airs or not.
Most laptops have a blue or other color substance on the screw threads to prevent vibration loosening them. You can use something like Loctite Blue threadlocker just a tiny bit applied as directions say to apply it and that should do the trick. Make sure and use the correct type of threadlocker as well........ too strong like the red or too much any type of it might be bad.
Using the correct size #00 phillips screw driver to tighten well might also have a positive effect. Wiha makes a nice ESD model #00
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